The present invention relates to pipettors for high throughput screening, and specifically to the use of a wash through pipettor for dispensing and aspirating.
Laboratories use pipettors to transfer liquids from one container to another. Automated pipettors for high throughput screening typically transfer liquid from one array of containers to another array of containers. The container arrays are most commonly 96 well or 384 well microtiter plates. The 96 array format uses 8 rows by 12 columns using 9 mm grid spacing. The 384 format uses twice the density with 16 rows by 24 columns using 4.5 mm grid spacing. When these pipettors are used, the tips need to be discarded or washed when changing liquids. The current method of washing the pipette tips is to aspirate wash fluid, and then dispense it into a waste container. The wash cycle is repeated many times to dilute the residue on the inside of the tips to an acceptable level.
Another concern with pipettors is dispense volume accuracy, especially at volumes less than 1 uL. When dispensing small volumes, the wetting and break-off behavior of the very small droplet at the tip is influenced by many factors. If the liquid prefers to wet to the target container (either dry or containing liquid already), then the liquid column in the pipette tip will experience a downward force as the liquid wets out into the target well. The amount of liquid dispensed is variable depending upon the wetting action as compared to the forces that will keep the rest of the liquid in the tip. Conversely, if the liquid is repelled from the target, then it may be pushed back up into the tip. The forces between the tips and containers are often random and variable. Current pipettors all have a sizeable volume of air above the liquid that acts as a coupling member between the piston and the liquid. The compressibility of the air and the vapor pressure therein create a weak coupling between the piston position and the fluid position.